1. Field of the Invention
This invention relates to the recording of monophasic action potentials (MAPs) and more particularly to a method and apparatus for measuring MAPs by contacting heart tissue with a small electrode under positive pressure.
2. Discussion of Related Art
Studies have been performed on tissues obtained from human hearts. It has been learned that a resting cardiac cell has a transmembrane voltage difference of about 90 mV. The inside of the cell is negative relative to the extracellular fluid and, upon cell stimulation, an action potential ensues. The action potential consists of five phases. Phase 0 is rapid depolarization, phase 1 is early repolarization, phase 2 is the plateau phase, phase 3 is a rapid repolarization to the diastolic transmembrane voltage, and phase 4 is the diastolic period. The time-voltage course of the action potential varies among different cardiac cell types.
To examine the time course of local electrical activity under experimental conditions in which microelectrode recordings are difficult or impossible to make, such as in the vigorous beating in-situ heart, investigators have often used suction electrodes. The signal obtained with suction electrodes is monophasic and, although of smaller amplitude, accurately reflects the onset of depolarization and the entire repolarization phase of transmembrane action potentials recorded from cells in the same vicinity. Suction electrodes have also been used in human subjects, but the potential for subendocardial damage and S-T segment elevation has limited its clinical use to short recording periods of two minutes or less. Because the shape and duration of the action potentials vary from site to site in the heart, longer recording time from a single endocardial site is needed to evaluate long-term MAP changes, such as heart rate effects over several basic cycle lengths or in response to pharmacologic interventions.